1. Field of the Invention
The present invention relates to a process of suspension polymerization for preparing a toner which is used for developing an electric latent image in an image-forming method such as electrophotography and electrostatic photography.
The present invention also relates to a toner prepared by the above-mentioned process.
2. Related Background Art
Toners are used for developing electric latent images or magnetic latent images in various image forming and recording processes.
Electrophotography, which is one of image forming processes, includes various methods such as shown in U.S. Pat. No. 2,297,691. In the electrophotography, a reproduced copy is prepared by forming an electric latent image generally with a photoconductive material; developing the latent image with a toner to form a toner image; optionally transferring the toner image onto an image-receiving material such as paper; and fixing the toner image on the image-receiving material or the like. Various methods are proposed for image development with a toner, and toner image fixation. Images are formed by methods suitable for respective image forming processes.
The toners used for these purposes are prepared generally by melt-blending a thermoplastic resin and a coloring material comprising a dye and/or a pigment to disperse the coloring material uniformly in the thermoplastic resin, and subsequently cooling, pulverizing, and classifying the mixture in a form of desired particle size.
This preparation process (pulverization process) has some limitations, such as in the range of selection of toner materials, even though the process gives toners of considerably high quality. For example, the dispersion of a coloring material in a resin should be sufficiently brittle to be pulverizable with an economically available pulverizing machine. Adjusting the required bittleness of the dispersion involves problems such that particles formed tend to have a broad particle size distribution, and that excessively fine particles are liable to be contained in the pulverized particles in a relatively large proportion. Moreover, such brittle material is subject to be further pulverized into fine powder in practical development use in image forming apparatuses such as copying machines.
In the pulverization method, fine solid particles such as a magnetic powdery material or a coloring powdery material cannot easily be dispersed uniformly in a resin. Since the degree of the dispersion of the fine solid particles relates to increase of fogging, and decrease of image density, sufficient care has to be paid to the degree of the dispersion. In some cases, a coloring material may be exposed at the broken section of a resin particle colored by a coloring material, causing variation of the developing characteristics of a toner.
To solve the problems involved in the toner prepared by pulverization method, manufacture of a toner by suspension polymerization is proposed in: Japanese Patent Publication No. 36-10231, British Patent No. 1583564, and U.S. Pat. Nos. 4,592,990 and 4,609,607. In this suspension polymerization, a polymerizable monomer and coloring material, (and additionally a polymerization initiator, a crosslinking agent, a charge controlling agent, and other additives as are necessary) are dissolved or dispersed uniformly to give a monomer composition, which is then dispersed in a continuous phase (e.g., an aqueous phase) containing a dispersion stabilizer by employing a suitable stirrer and is subjected to polymerization reaction to give a toner having a desired particle size.
In the preparation of a toner by suspension polymerization, a pigment or a dye is used as the coloring material. Since some coloring materials remarkably inhibit polymerization, applicable coloring materials are considerably limited in the suspension polymerization method, while in the pulverization method, polymerization-inhibiting coloring materials are applicable. In particular, dyes tend significantly to inhibit polymerization, so that it has been difficult to prepare by suspension polymerization a toner having a desired color and being excellent in development characteristics.